Cancer has been shown to be a product of both dominant-acting genes (oncogenes) and recessive genes (tumor suppressor genes, or anti-oncogenes) impinging upon normal cells. The development of retinoblastomas in children, and some types of sporadic cancers of adults, results from inactivation of the tumor suppressor gene Rb-1, which encodes a nuclear phosphoprotein, denoted pRb. The specific alteration of Rb-1 in tumors indicates that the normal function of pRb is to provide a growth-inhibitory signal to tumor cells, and that its absence contributes to tumor development. Current information suggests the hypothesis that pRb mediates this growth-inhibitory effect by binding to a nuclear target protein, and that normal growing cells override this signal by phosphorylation of pRb, whereupon pRb is dissociated from its nuclear target. To test this hypothesis we propose: 1. To probe the structure of pRb by saturation mutagenesis of pRb expression vectors. The mutated forms of pRb will be assessed for phosphorylation and other biochemical properties, and also tested for biological function using a unique assay for pRb biofunction. 2. To model regulation of pRb association with nuclei using pRb overexpressed in vaccinia viral vectors, and purified by immunoaffinity chromatography. The effect of dephosphorylation of pRb on nuclear association will be tested, as well as that of phosphorylation using immunopurified protein kinases suspected of phosphorylating pRb in vivo. 3. Development of methodologies to identify and clone the genes for proteins that bind to pRb, using purified pRb for solid- or liquid-phase detection. Such proteins will be candidates for the nuclear target of pRb. Elucidation of the mechanism of pRb tumor growth-inhibition and identification of proteins to which pRb transmits the inhibitory signal may provide opportunities to interfere with malignant growth therapeutically.